Newer (and future) mobile terminals have to support a large number of frequency bands and cellular system standards. A solution for a terminal (also referred to as a user equipment or UE) that is efficient both from a cost and a size point of view and one which supports a number of frequency bands is to permit only half duplex operation in some of the bands.
Half duplex operation provides either transmission or reception at an instant in time—that is, it does not permit simultaneous transmission and reception. By utilizing half duplex operation, the need for a large duplex filter can be obviated. This results in a reduction in the cost associated with the large duplex filter. As duplex filters suffer a significant front-end power loss, the use of half-duplex (HD) operation also leads to a gain in terms of UE power consumption—this is especially true for high output powers.
The scheduling of half-duplex UEs in systems such as LTE and GSM are illustrated in FIG. 1. The base station (or eNodeB) 110 operates in a full duplex mode. The eNodeB can transmit and receive simultaneously as illustrated in FIG. 1. The UEs 120, on the other hand, operate in a half duplex mode. That is, UEs 120 can either receive or transmit but cannot perform both functions simultaneously as illustrated in FIG. 1. Depending on the standard, the allocation of subframes (or time slots) for uplink (UL) or downlink (DL) at the UE can be allocated in a more or less dynamic manner. In GSM, the uplink-downlink allocation is relatively fixed while in LTE, the allocation is dynamic as described further below.
The time-domain structure in LTE transmissions (both uplink and downlink) is made up of subframes that are each one millisecond (1 ms) in duration. For each subframe, a scheduler in the eNodeB controls which UEs should receive data in the downlink and/or transmit data in the uplink. Multiple UEs can be addressed in the same subframe. This could be accomplished by using, for example, separate parts of the frequency spectrum for transmission.
In frequency division duplexing (FDD), separate channels are utilized for uplink and downlink. Uplink is transmission from a UE (or terminal) to eNodeB (or base station) such as from UE 120 to eNodeB 110 in FIG. 1. Downlink is reception by UE from eNodeB such as by UE 120 from eNodeB 110 in FIG. 1.
In full-duplex FDD (frequency division duplexing), a UE can both transmit and receive in a given subframe (i.e. simultaneously). In half-duplex operation, a UE can only transmit or receive but cannot transmit and receive simultaneously. In LTE, half-duplex FDD is implemented as a scheduler constraint. It is up to the scheduler (in eNodeB) to ensure that a single UE is not scheduled simultaneously (from a time point of view) in uplink and downlink. Therefore, from a UE perspective, subframes are dynamically used for uplink or downlink.
For half-duplex FDD in LTE, a UE receives in the downlink unless it is transmitting in the uplink. In other words, unless a UE is transmitting, it is in a receiving mode. The UE can transmit, among other things, the following: data, hybrid-Automatic Repeat reQuest (hybrid ARQ or HARQ) acknowledgements triggered by a downlink transmission, channel-quality reports (CQI), scheduling requests (SRs) or random access attempts (RACH) for example.
An example of half-duplex operation in LTE as seen from a UE perspective is illustrated in FIG. 2. In subframe 201, the UE is explicitly scheduled in the uplink. Therefore, the UE cannot receive data in the downlink in the same subframe (i.e. subframe 201). The uplink transmission implies (or, can result in) the reception of a hybrid-ARQ acknowledgement or an ACK/NACK from eNodeB for a packet received from the UE in the downlink four subframes later for example (i.e. subframe 205 for example). As a result, the UE cannot be scheduled in the uplink in this subframe (since a HARQ is sent from eNodeB in subframe 205 and received by the UE in the downlink).
Similarly, when the UE is scheduled to receive data in the downlink in subframe 207, a hybrid-ARQ (HARQ) acknowledgement needs to be transmitted in uplink subframe 211, preventing downlink reception in this subframe (i.e. 211).
The scheduler can exploit this scheme by scheduling downlink data in four consecutive subframes and uplink transmission in the four next subframes (when the UE needs to transmit hybrid-ARQ acknowledgements) and so on. A HARQ is always sent 4 ms after a data packet. If the UE transmits UL data and a HARQ on a 4 ms old downlink data simultaneously, the information is multiplexed together in an uplink transmission that includes both HARQ and data. Consequentially, at most half of the time can be used in the downlink and half in the uplink. This may be better described as the asymmetry in half-duplex FDD being 4:4.
In order to facilitate scheduling in the LTE uplink, the UE should have the ability to request uplink resources from eNodeB for data transmission (from UE to eNodeB). This is addressed by allowing the UE to transmit a scheduling request (SR) at certain time instants. The SR and RACH time instances and periodicities are sent by eNodeB. The periodicity (5 or 10 or 40 ms for example) is the frequency; the time instant is similar to a phase that specifies which subframe should be used by the UE for a transmission.
Upon detection of an SR from a given UE, the scheduler can allocate resources to the UE (for data transmission). For providing connection to a cell, LTE (as any cellular system) has a mechanism for the UE to perform a random access (RACH) in certain subframes. RACH instants are muxed (multiplexed) together with data, i.e. during some UL sub frames some parts of the frequency bandwidth is allocated to RACH signals. The same holds for SR. Upon reception of the random access request, eNodeB initiates a procedure where the UE identifies itself and connects to the cell. As long as the UE does not have a good uplink (UL) time synchronization, a request should be made by a RACH (if no UL requests have been made during the last 10-30 seconds for example). If the UL time synchronization is known (UL transmissions have occurred less than 10 seconds earlier for example), the SR should be used.
As described, half-duplex provides certain advantages such as in cost and in size. It does also have a drawback in that its maximum allowed throughput is reduced due to not having the possibility of allocating all subframes to UL and/or DL. However, for LTE and future cellular systems supporting data rates up to and above 100 MB/s, half-duplex terminals still can reach high data rates (such as data rates >>20 Mb/s for example). This is sufficient in many cases especially when the reduced cost of an HD UE is taken into account.
There is an inherent problem in using HD UEs in full duplex systems like LTE due to the random nature of scheduling requests and random access by the terminal (or, UE). As described above, the uplink retransmissions of data (from UE to eNodeB) and/or hybrid-ARQ acknowledgements related to downlink transmissions (from eNodeB to the UE) are explicitly controlled by the scheduler (in eNodeB). Random access and scheduling requests, on the other hand, are initiated autonomously by the UE.
The eNodeB does not know in advance if any of RACH or SR will be present. Therefore, there is a risk or a possibility of a DL packet being transmitted by eNodeB to the UE at the same time as the RACH and/or SR from the HD UE is received by eNodeB. However, each is UE is allocated a RACH and SR transmission (and also CQI reporting) pattern via higher layer signalling. Therefore, possible RACH, CQI reports and SR instants are known to eNodeB. By properly configuring DL allocations to HD UEs, DL (download) packets could in principle be avoided at the subframes allocated to the UL (upload) instants.
RACH and SR might occur on a relatively infrequent basis. By avoiding DL allocation every 2-20 ms for example (when RACH/SR is allowed for that particular HD UE), the UL and DL collisions at the UE can be avoided.
If a RACH/SR occurs at a rate of every 100 ms or even more seldom, it is a significant waste of resources to forbid DL transmissions every 5 or 20 ms for example. That is, for collisions that might happen on a 100 ms to several second basis, avoiding DL transmissions every 5 or 10 or 20 ms for example, is a significant waste of DL resources. In a typical scheduler implementation, such waste of DL resources is not tolerated. The collision of DL data with SR or RACH will be handled by HARQ (Hybrid automatic repeat request) or higher layer transmissions. This, however, has a negative impact on the HD terminal throughput.
A need exists, therefore, for half duplex UEs to take into account the problem of SR/RACH collision with DL data.